One of the causes of wear in an internal combustion engine is corrosion of the metal surfaces of the engine, particularly lead and copper metal surfaces caused by the action of various corrosion-promoting compounds which accumulate in the crankcase of the engine. The corrosion-promoting compounds present in the crankcase are principally weak organic acids which may result from nitration and oxidation of the lubricating oil due to contamination by blow-by gases and exposure of the lubricant to high temperatures in the piston and ring zones. Regardless of the source of the corrosion-promoting compounds, it is important to protect the engine from the deleterious action of such compounds and thereby reduce engine wear. For the purpose of preventing corrosivity by these compounds on the various engine parts, it is necessary to incorporate dispersants, detergents, and corrosion inhibitors in the lubricating oil composition, to limit the formation of corrosion products and protecting metal surfaces.
Historically, crankcase oils usually contain ash from detergents, anti-wear products with metals, e.g., Zn, Ca, and the like. Although these metal-containing organic compounds have corrosion inhibition activity as well as detergency, they form undesirable ash deposits in the engine. Ash deposits can lower engine performance by fouling spark plugs, contributing to combustion chamber deposits that cause preignition, or facilitating carbon deposits in two-cycle engine ports and thus are undesirable in many applications. However, it is quite challenging to control corrosion without metal-containing additives like detergents and anti-wear agents. Ashless lubricants would have the advantage of reducing combustion chamber deposits. For example, U.S. Pat. No. 5,320,765, issued on Jun. 14, 1994 to Fetterman, Jr. et al., disclose marked reductions in diesel engine carbon deposits with lubricating oil compositions containing a high molecular weight ashless dispersant, oil soluble antioxidants, and oil soluble dihydrocarbyl dithiophosphate.
Currently, ashless products are created with metal pacifiers such as terephthalic acid (TPA) to protect the metal surface. However, as highly effective as TPA is in corrosion inhibition, it is very oil insoluble and may contribute to deposits in the engine at low engine temperatures. Thus, it would be desirable to attain the corrosion inhibition performance comparable to TPA but devoid of the insoluble deposit problem associated with TPA.
It is also desirable to minimize the amount of phosphorus in lubricants. Although phosphorus does not contribute to ash, it can lead to poisoning of catalysts in pollution control devices such as emission catalysts or traps when amounts of phosphorus make their way into the exhaust system. Exemplary of references directed to the reduction in phosphorus-containing lubricant additives are U.S. Pat. Nos. 4,147,640, 4,330,420, and 4,639,324.
Combined dispersant-corrosion inhibitors are known in the art. For instance, U.S. Pat. No. 3,287,271, issued Nov. 22, 1966 to Stuart, discloses a novel composition which provides both corrosion inhibition and detergency by combining a polyamine with a high molecular weight succinic anhydride and then contacting the resulting product with a dicarboxylic acid, having the carboxyl groups separated by at least three annular carbon atoms.
U.S. patent application Ser. No. 09/015,801, filed Jan. 29, 1998, discloses a lubricating oil composition for internal combustion engines giving improved soot dispersancy. That lubricating oil composition has a mixture of borated and carbonated polyalkylene succinimides derived from different molecular weight polyalkylenes. The molecular weight of the polyalkylenes from which the carbonated polyalkylene succinimide is derived is at least 300 greater than the molecular weight of the polyalkylenes from which the borated polyalkylene succinimide is derived. The lubricating oil compositions of the examples contained in this application included significant portions of metal detergents and zinc dithiophosphates creating high ash content and high phosphorus content formulation.